Contacts to semiconductor devices are generally formed by providing openings in the silicon oxide layer, which is generally over the surface of the silicon substrate, to the area of the substrate surface where contact is to be made. A conductive contact material is then deposited over the silicon oxide layer and in the openings to coat the exposed area of the substrate surface. The contact material may be a metal, such as aluminum, or conductive polycrystalline silicon. If the contact material is aluminum, the device is then generally heated to alloy the contact metal with the silicon of the substrate to form a good ohmic contact. In semiconductor devices which have multi-levels of conductors, such as integrated circuits, contacts are formed between the various levels by the outer level extending through openings in the insulating layer between the levels and making an ohmic contact with the next innermost level.
However, it has been found that the ohmic contact between the conductive layer and the silicon substrate or between various levels of the conductors frequently have higher than desired contact resistance. Where a semiconductor device has a large number of contacts, some of the contacts may have satisfactory low resistance while others may have poor or less than desirable contact resistance. One reason for the poor ohmic contact is the presence of a thin layer of native oxide on the surface of the silicon substrate or the lower level conductor. When the opening is formed in the silicon oxide layer to expose the surface of the silicon substrate or the lower level conductor, the surface of the silicon substrate or the lower level conductor is exposed to the atmosphere and a thin layer of native oxide is formed thereon. Although the exposed surface is generally cleaned by a suitable etchant, the surface is exposed to the atmosphere for a sufficient time between the cleaning and the deposition of the contact material to allow a thin layer of native oxide to form thereon. This native oxide layer acts as an insulator to increase the contact resistance between the contact material and the surface of the silicon substrate or the lower level conductor. When the contact material is a metal which is applied by evaporation or sputtering, another reason for the poor contact resistance is the oxidation of the initially arriving metal atoms during the deposition of the metal. The oxidation of the metal ions is a result of the outgassing of moisture when the shutter is first opened in the evaporator or sputtering system.